Natural sponges have excellent water retention and physical, mechanical and chemical resistance qualities when used with normal household and industrial cleaning materials. Besides being somewhat scarce and expensive, natural sponges have irregular shapes and dimensions and thus are not selected for use in many commercial applications where standard shapes and dimensions are required.
One of the synthetic sponge-type materials made to replace natural sponge is regenerated cellulose. Regenerated cellulose can be commercially manufactured with desirable standardized shapes and dimensions. Synthetic cellulose sponges have sufficient water retention properties for common uses, but they also have a high production cost and inferior physical, mechanical and chemical resistance properties when they are used with common household maintenance products.
Less expensive sponge-type alternatives having better physical and chemical resistance have been developed. These include synthetic open celled foam sponges made of natural rubber or of a synthetic rubber polymer latex, principally from polyvinyl chloride, polyamide and polyurethane. These foams can be commercially manufactured at reasonable cost and have acceptable physical and chemical resistance, but they are generally hydrophobic and consequently have only a small water retention capacity, thus making them generally unacceptable for satisfactory wiping.
Such synthetic hydrophobic foam materials have been rendered hydrophilic by superficial coating of a hydrophilic substance such as clay. While synthetic sponge-type materials having water retention characteristics close to those of cellulose were obtained by this technique, the clay coating is only temporary and it soon disappears after a few rinses.
While not previously applied to hydrophobic foams to improve wiping properties, various methods are known to facilitate the slow diffusion of volatile or soluble substances, such as perfumes or medicines, from other types of articles such as a lozenge, granule, wrist watch, ear ring, plaque or strip. These involve the formation of absorbent hydrophilic inclusions in a hydrophobic support. This has been accomplished by polymerization in situ of monomers which on polymerization produce an absorbent material in the hydrophobic substrate. This method is described in the French Patent No. 2 250 793 (published June 6, 1975) and its addition certificate No. 2 348 238 (published November 10, 1977) which describes a process in which monomer (polymerizable to an absorbent reticulum) is polymerized on a hydrophobic substrate and the polymerization is controlled by ionic or ultrasonic radiation, or by immersion of the hydrophobic substrate into a solution containing the monomers, to produce hydrophilic inclusions in the hydrophobic substrate. No disclosure is made in these references of making articles having the required sponge-type properties of a wiping article and the disclosed articles appear to have a rigid structure rather than that of a flexible sponge.
The application to sponge-type articles of the aforementioned disclosed technique does not readily provide the present invention since these references fail to appreciate the physical property requirements that are needed to produce a commercially useful sponge-type article having excellent wiping properties. On the contrary, a systematic study reveals important differences from one material to the other, and an absence of any relationship between the capacity to retain water by capillary action of the same treated and untreated supports, and their structural characteristics, such as cell diameter, air-permeability and specific surface. Thus, surprisingly the same absorbency treatment does not result in a comparable improvement in the water retention characteristics of hydrophobic open celled foams having different physical structures.